Nitride semiconductor device and method for fabricating the same

ABSTRACT

A nitride semiconductor device includes: a first nitride semiconductor layer; a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a wider band gap than the first nitride semiconductor layer; and a third nitride semiconductor layer formed on the second nitride semiconductor layer. A region of the third nitride semiconductor layer located below the gate electrode is formed with a control region having a p-type conductivity, and a region of the third nitride semiconductor layer located between the gate electrode and each of the source electrode and the drain electrode is formed with a high resistive region having a higher resistance than the that of the control region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 on Patent Application No. 2006-278913 filed in Japan on Oct. 12, 2006, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(a) Fields of the Invention

The present invention relates to nitride semiconductor transistors and their fabrication methods. In particular, the present invention relates to normally-off type nitride semiconductor transistors applicable to power switching devices, and their fabrication methods.

(b) Description of Related Art

In recent years, field effect transistors (FET) made of gallium nitride (GaN)-based materials have been widely investigated for high frequency devices and high power devices. As the GaN-based nitride semiconductor, not only GaN but also aluminum nitride (AlN), indium nitride (InN), and the like are used. Since these nitride semiconductors can form various types of compound materials, they can form heterojunctions like conventional arsenic-based semiconductors such as gallium arsenide (GaAs). However, in the heterojunction of the nitride semiconductor, even though impurities are not added in the nitride semiconductors intentionally, spontaneous polarization and piezoelectric polarization produces high sheet carrier densities at the interface thereof. As a result of this, most of GaN-based heterojunction FETs exhibit depletion (normally-on) characteristics, and it is difficult to exhibit enhancement (normally-off) characteristics. Aiming at the practical applications, normally-off operation is strongly desired for FETs fabricated using the nitride semiconductor in order to make the GaN devices compatible with currently used Si-based power MOSFETs/IGBTs for safe operation.

Conventional structures of normally-off type FETs using nitride semiconductors include: a structure in which the thickness and/or the Al mole fraction of an AlGaN layer are decreased; a structure in which a gate region is partially recessed to shift a threshold voltage positively (see, for example, T. Kawasaki et al., “Solid State Devices and Materials 2005 tech. digest”, 2005, pp. 206); the structure in which a FET is fabricated on the (10-12) plane of a sapphire substrate to prevent a polarization electric field from being generated in the crystal growth direction of the nitride semiconductor (see, for example, M. Kuroda et al., “Solid State Devices and Materials 2005 tech. digest”, 2005, pp. 470); and the like.

As a promising structure for obtaining the normally-off type FET, a junction field effect transistor (JFET) is proposed in which a p-type GaN layer is formed as gate (see, for example, Japanese Unexamined Patent Publication No. 2005-244072). In a conventional JFET, a buffer layer, a channel layer of undoped GaN, and a barrier layer of n-doped AlGaN are sequentially formed over a substrate. On the barrier layer, a source electrode and a drain electrode are formed separately. Between the source electrode and the drain electrode, a gate electrode is formed with a p-GaN layer.

In the conventional JFET structure, piezoelectric polarization generated at the heterointerface between the channel layer of undoped GaN and the barrier layer of AlGaN is cancelled by piezoelectric polarization generated at the heterointerface between the barrier layer of AlGaN and the p-type GaN layer. This structure decreases the concentration of two-dimensional electron gas in an area just below the gate, whereby the normally-off characteristics can be obtained. In addition, a pn junction gate having a higher built-in potential than a Schottky junction, provides an advantage that even though a positive gate voltage is applied, a gate leakage current can be maintained at lower level.

However, unlike a silicon semiconductor, it is difficult to form p-type or n-type conductive areas selectively by ion implantation or thermal diffusion in the nitride semiconductors. Thus, a promising approach to form the p-type gate is, for example, a selective regrowth of a p-type nitride semiconductor just only at the gate area. In such selective regrowth, however, silicon which acts as a n-type impurity in nitride semiconductor is easily segregated at the regrowth interface. So it is difficult to obtain a good pn junction in this method.

Another promising approach to form the p-type gate is a selective etching method in which a p-type nitride semiconductor is deposited and etched locally remaining the portions of the p-type nitride semiconductor layer only at the gate area. This method, however, has a serious problem that the etching of the p-type nitride semiconductor layer causes electrical damages at the channel region and it is difficult to obtain good electrical properties of FETs. When the channel region is damaged by etching, the carrier concentration in the channel region between the gate and the drain decreases to cause a reduction in drain current. Moreover, trapping of electrons into defects created on the surface of the channel region or other damage-induced influences inhibit a fast switching operation, that is, a so-called current collapse occurs.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the conventional problems described above, and to provide a nitride semiconductor device which has a large operating current with normally-off operation and excellent switching characteristics.

To attain the above object, in the present invention, a nitride semiconductor device is designed so that a control region with a p-type impurity activated therein is provided in a region below a gate electrode.

To be more specific, a nitride semiconductor device according to the present invention is characterized in that it includes: a first nitride semiconductor layer; a second nitride semiconductor layer; a third nitride semiconductor; and a gate electrode. The first nitride semiconductor layer is formed over a substrate. The second nitride semiconductor layer is formed on the first nitride semiconductor layer and has a wider band gap than the first nitride semiconductor layer. The third nitride semiconductor layer is formed on the second nitride semiconductor layer and includes a control region with a p-type conductivity and a high resistive region formed around the control region and having a higher resistance than that of the control region. The gate electrode is formed on the control region.

With the nitride semiconductor device of the present invention, a forward bias voltage with respect to the channel region is applied to the control region to inject holes into the channel region. The injected holes induce generation of additional electrons in the channel region, so that the carrier concentration in the channel region is increased significantly. As a result of this, the normally-off type nitride semiconductor transistor having an increased operating current is obtained. Furthermore, the third nitride semiconductor layer is formed on the second nitride semiconductor layer, in which the distance between the surface and the channel region is enlarged and it becomes negligible to receive the influence of the surface. As a result of this, a decrease in carrier concentration in the channel region and occurrence of current collapse are prevented.

A method for fabricating a nitride semiconductor device according to the present invention is characterized in that it includes: the step (a) of forming, over a substrate, a first nitride semiconductor layer, a second nitride semiconductor layer, and a third nitride semiconductor layer in this order from bottom to top, the second nitride semiconductor layer having a wider band gap than the first nitride semiconductor layer; the third nitride semiconductor layer containing a p-type impurity combining with hydrogen; the step (b) of forming a control region having a p-type conductivity by hydrogen desorption from a portion of the third nitride semiconductor layer; the step (c) of forming a gate electrode on the control region; and the step (d) of forming a source electrode and a drain electrode on both sides of the gate electrode, respectively, the source electrode and the drain electrode being electrically connected to a channel region.

With the method for fabricating a nitride semiconductor device according to the present invention, in selectively forming the control region with p-type conductivity, the third nitride semiconductor layer does not have to be etched. Therefore, the second nitride semiconductor layer is prevented from being damaged by etching, and thereby the nitride semiconductor device of this invention shows the large operating current and fast switching operation characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a nitride semiconductor transistor according to a first embodiment of the present invention.

FIGS. 2A and 2B show energy band structures of the nitride semiconductor transistor according to the first embodiment of the present invention. FIG. 2A shows the energy band in the control region, and FIG. 2B shows the energy band in a region other than the control region.

FIGS. 3A and 3B are graphs showing the characteristics of the nitride semiconductor transistor according to the first embodiment of the present invention. FIG. 3A is a graph showing the drain current and the gate voltage characteristics, and FIG. 3B is a graph showing the drain current and the drain voltage characteristics.

FIG. 4 is a graph showing the correlation between the temperature of a thermal treatment and the resistivity of a nitride semiconductor layer containing a p-type impurity in the nitride semiconductor transistor according to the first embodiment of the present invention.

FIG. 5 is a cross sectional view showing a nitride semiconductor transistor according to a second embodiment of the present invention.

FIG. 6 is a graph showing the drain current and the drain voltage characteristics of the nitride semiconductor transistor according to the second embodiment of the present invention.

FIG. 7 is a cross sectional view showing a nitride semiconductor transistor according to a third embodiment of the present invention.

FIG. 8 is a graph showing the correlation between the temperature of a thermal treatment and the resistivity of a nitride semiconductor layer containing a p-type impurity in the nitride semiconductor transistor according to the third embodiment of the present invention.

FIG. 9 is a graph showing the correlation between the hydrogen concentration in a hydrogen diffusion film and the resistivity of the nitride semiconductor layer containing a p-type impurity in the nitride semiconductor transistor according to the third embodiment of the present invention.

FIG. 10 is a cross sectional view showing a nitride semiconductor transistor according to a fourth embodiment of the present invention.

FIG. 11 is a band diagram showing an energy band structure of the nitride semiconductor transistor according to the fourth embodiment of the present invention.

FIG. 12 is a graph showing the drain current and the gate voltage characteristics in the nitride semiconductor transistor according to the fourth embodiment of the present invention.

FIG. 13 is a cross sectional view showing a nitride semiconductor transistor according to a fifth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a cross-sectional structure of a nitride semiconductor transistor according to the first embodiment of the present invention.

Referring to FIG. 1, a substrate 11 is made of silicon and its principal plane is the (111) plane. A buffer layer 12 of AlN having a thickness of 100 nm is formed on the principal plane of the substrate 11.

A first nitride semiconductor layer 13 of undoped GaN having a thickness of 2 μm and a second nitride semiconductor layer 14 of undoped Al_(0.15)Ga_(0.85)N having a thickness of 25 nm are formed on the buffer layer 12. A channel region of the transistor which will be described later is formed in the interface region between the first nitride semiconductor layer 13 and the second nitride semiconductor layer 14.

On the second nitride semiconductor layer 14, a third nitride semiconductor layer 15 is formed which is made of Al_(0.15)Ga_(0.85)N doped with magnesium (Mg) and has a thickness of 200 nm. The third nitride semiconductor layer doped with Mg is composed of: a control region 15 a in which Mg is activated to exhibit a p-type conductivity; and a high resistive region 15 b in which Mg is not activated to offer a higher resistance than the control region 15 a. The control region 15 a is formed in the shape of, for example, stripes each having a width of 1 μm.

A hydrogen diffusion barrier film 16 of AlN having a thickness of 100 nm is formed on the third nitride semiconductor layer 15. The hydrogen diffusion barrier film 16 has an opening for exposing the control region 15 a. On the hydrogen diffusion barrier film 16, a gate electrode 20 is formed to cover the opening. The gate electrode 20 is made of palladium (Pd), and comes into ohmic contact with the control region 15 a.

Both sides of the gate electrode 20, a source electrode 18 and a drain electrode 19 are formed, respectively. The source electrode 18 and the drain electrode 19 are each made of titanium (Ti) and aluminum (Al), and formed on the partly recessed first nitride semiconductor layer 13 through the hydrogen diffusion barrier film 16, the high resistive region 15 b of the third nitride semiconductor layer 15, and the second nitride semiconductor layer 14. With such a structure, the source electrode 18 and the drain electrode 19 can obtain good ohmic contact characteristics with a channel region at the heterointerface between the first nitride semiconductor layer 13 and the second nitride semiconductor layer 14. The separation length from the control region 15 a to the drain electrode 19 is preferably set at 3 μm or greater in order to increase the drain breakdown voltage.

Note that only one transistor is illustrated in FIG. 1, but multi transistors could be arrayed in which unit structures isolated by an isolation region (not shown) are formed.

The following description will be made of the principle of operation of the transistor according to the first embodiment. FIGS. 2A and 2B show energy bands of the transistor according to the first embodiment. FIG. 2A shows the energy band in the control region, and FIG. 2B shows the energy band in a region other than the control region.

Referring to FIGS. 2A and 2B, in both of the control region and the region other than the control region, the conduction band has a discontinuity at the heterointerface between the first nitride semiconductor layer 13 and the second nitride semiconductor layer 14. In the region other than the control region, as shown in FIG. 2B, at the heterointerface the conduction band is located at a lower level than the Fermi level, and thus 2DEG is created. The p-type impurity contained in the third nitride semiconductor layer 15 in the region other than the control region is not activated and thus the third nitride semiconductor layer 15 in the region other than the control region acts as high resistive layer. On the other hand, in the control region, as shown in FIG. 2A, the p-type impurity contained in the third nitride semiconductor layer 15 is activated and thus the energy levels of the second nitride semiconductor layer 14 and the first nitride semiconductor layer 13 are lifted up. As a result of this, the conduction band at the heterointerface between the first nitride semiconductor layer 13 and the second nitride semiconductor layer 14 is located at a higher level than the Fermi level. Therefore, when no bias voltage is applied to the control region, 2DEG is not created at the heterointerface. This results in obtaining the normally-off characteristics of nitride semiconductor transistors.

For example, the number of electrons in the 2DEG layer created in the interface region between 2 μm-thick undoped GaN and 25 nm-thick undoped Al_(0.15)Ga_(0.85)N is about 1×10¹³ cm⁻². The Mg concentration doped into the third nitride semiconductor layer 15 is set at about 1×10¹⁹ cm⁻³, and the carrier concentration in the control region 15 a is set at about 1×10¹⁸ cm⁻³. In this condition, when the thickness of the control region 15 a is set at 200 nm, a p-type carrier concentration of about 2×10¹³ cm⁻² can be provided. The number of holes contained in the control region 15 a can be more than the number of electrons generated in the channel region when the control region 15 a does not exist, and thereby the normally-off type nitride semiconductor transistor can be obtained.

In the transistor of the first embodiment, holes are injected into the channel region by forward biasing the control region with respect to the channel region. Since the hole mobility of the nitride semiconductor is much lower than the electron mobility thereof, the holes injected into the channel region hardly contribute as carriers for drain currents. The holes injected from the control region generate, in the channel region, electrons as many as the number of injected holes. This makes it possible to dramatically increase a current flowing in the channel region, and thereby a transistor with a large operating current can be obtained.

Furthermore, by designing the band gap in the control region larger than the band gap in the channel region, the amount of holes to be injected from the control region into the channel region can be made greater than the amount of electrons to be injected from the channel region into the control region. This makes it possible to further efficiently increase the carrier concentration in the channel region.

As the mobility of holes injected into the channel region is much smaller than that of electrons, the effect of generating electrons in the channel region is increased. Then, the holes act like as donor ions. That is to say, the carrier concentration can be modulated in the channel region, so that a normally-off type nitride semiconductor transistor with a large operating current can be obtained.

The transistor structure of the present invention is similar to that of a junction field effect transistor (JFET). However, in the point that carriers are injected intentionally, it operates by an entirely different principle of operation from the JFET in which a gate electric field modulates carriers in the channel region.

FIGS. 3A and 3B are graphs showing the characteristics of the transistor according to the first embodiment. FIG. 3A shows the drain current and the gate voltage characteristics, and FIG. 3B shows the drain current and the drain voltage characteristics. As shown in FIGS. 3A and 3B, the obtained threshold voltage is 1 V, and the transistor exhibits normally-off characteristics. Even when a positive bias of 5 V or higher is applied to the gate electrode, no significant gate-leakage current is observed and a maximum drain current of about 300 mA/mm is obtained.

In a nitride semiconductor transistor, a thin AlGaN layer with a thickness of 20 to 30 nm is formed on a GaN layer, and the heterointerface between the GaN layer and the AlGaN layer serves as a channel region. Also, the AlGaN layer is provided as the surface of the transistor. With this structure, the distance from the surface of the transistor to the channel region becomes very short, and thus the transistor tends to be affected by electron trapping resulting from crystal defects in the surface of the nitride semiconductor or the like. If the transistor is affected by the electron trapping, on-off switching of the current of the transistor cannot be carried out in a short time in the on-off switching operation. This will cause a so-called current collapse in which a current capable of being activated greatly decreases, which inhibits a fast switching operation. However, the transistor of the first embodiment has the third nitride semiconductor layer formed on the second nitride semiconductor layer. Therefore, an adequate distance can be secured between the surface of the transistor and the channel region, which in turn provides the effect of preventing the current collapse.

Hereinafter, a method for fabricating a transistor according to the first embodiment will be described. First, by a metal organic chemical vapor deposition (MOCVD) method, the buffer layer 12 of AlN, the first nitride semiconductor layer 13 of undoped GaN, and the second nitride semiconductor layer 14 of undoped Al_(0.15)Ga_(0.85)N are sequentially grown on the substrate 11. Thereafter, the third nitride semiconductor layer 15 of Al_(0.15)Ga_(0.85)N is grown with Mg doped as a p-type impurity. Then, the hydrogen diffusion barrier film 16 of AlN is formed.

Next, the hydrogen diffusion barrier film 16 is selectively etched to form an opening for exposing the third nitride semiconductor layer 15.

Subsequently, in a nitrogen gas (N₂ gas) atmosphere, a thermal treatment is performed at 800° C. for about 20 to 60 minutes. Thereby, in the portion of the third nitride semiconductor layer 15 exposed from the opening, Mg contained therein is activated to form the control region 15 a having a p-type conductivity. Note that instead of N₂ gas, an inert gas such as argon may be used.

When, for example, Mg is doped as a p-type impurity during crystal growth for the nitride semiconductor layer by a MOCVD method, Mg is introduced into the nitride semiconductor. However, Mg introduced into the nitride semiconductor is inactivated in which it is combined with a hydrogen atom. When a thermal treatment is performed in an inert gas atmosphere at 800° C. for about 20 to 60 minutes, the hydrogen atom is desorbed from Mg. Then, the Mg is activated, and thus the nitride semiconductor layer shows p-type conductivity.

FIG. 4 shows the correlation between the temperature of the thermal treatment and the resistivity in the case where an Al_(0.15)Ga_(0.85)N layer doped with Mg to have a concentration of about 1×10¹⁹ cm⁻³ is subjected to a thermal treatment. The thermal treatment was performed in a nitrogen atmosphere for 20 minutes. As shown in FIG. 4, in the case of forming no hydrogen diffusion barrier film of AlN, the resistivity of the Mg doped Al_(0.15)Ga_(0.85)N layer decreases with an increasing temperature of the thermal treatment, and becomes almost constant at around 800° C. On the other hand, in the case of forming the hydrogen diffusion barrier film of AlN, the resistivity of the Mg doped Al_(0.15)Ga_(0.85)N layer is almost constant even though the temperature of the thermal treatment is increased. This indicates the fact that formation of the hydrogen diffusion barrier film blocks hydrogen desorption from Mg to keep Mg in-activated state.

In the first embodiment, the thermal treatment is performed in the state in which the third nitride semiconductor layer 15 with Mg doped therein is covered with the hydrogen diffusion barrier film 16 having the opening. Thus, in the portion of the third nitride semiconductor layer 15 exposed from the opening, hydrogen atoms are desorbed from Mg, so that the p-type impurity is activated. However, in the portion of the third nitride semiconductor layer 15 covered with the hydrogen diffusion barrier film 16, less or no hydrogen desorbs from Mg, so that Mg is not activated. As a result of this, selective formation can be made of: the control region 15 a in which the p-type impurity is activated to exhibit a p-type conductivity; and the high resistive region 15 b in which the p-type impurity is not activated to offer a high resistive layer.

As described above, with the method for fabricating a transistor according to the first embodiment, the p-type control region can be formed locally without etching the nitride semiconductor, and by a simple process, the transistor gate can be formed without any damages.

It is desirable that the high resistive region have a higher resistance. However, it is required that it has at least a higher resistance than the control region, and it is preferable that the difference between the resistivity of the high resistive region and the resistivity of the control region should be equal to or more than four orders of magnitude.

In the first embodiment, a silicon substrate whose principal plane is the (111) plane is used as a substrate. Alternatively, a substrate made of silicon carbide (SiC), gallium nitride (GaN), sapphire, or the like may be employed thereas. The principal plane of the substrate may have any plane direction as long as it can provide good crystal quality of nitride semiconductor layer. The second nitride semiconductor layer 14 and the third nitride semiconductor layer do not have to have the same Al composition, and the third nitride semiconductor layer may be made of GaN without Al. Description has been made of the example where the first nitride semiconductor layer and the second nitride semiconductor layer are undoped, but the case where the second nitride semiconductor layer contains an n-type impurity such as Si also exerts the same effect.

The case where the hydrogen diffusion barrier film 16 is made of AlN has been shown in this embodiment, but instead of AlN, aluminum oxide (Al₂O₃), silicon nitride (SiN), or the like may be employed. In the case where a thermal treatment step of exposing the transistor to a temperature of 400° C. or higher is not carried out after formation of the control region 15 a, the hydrogen diffusion barrier film 16 may be removed after formation of the control region 15 a.

The case where the gate electrode 20 is made of Pd has been shown in this embodiment. However, it is sufficient that it can be brought into ohmic contact with the control region, so that nickel or the like may be used thereas. Or, a stacked film made of multi layers of metal materials may be used thereas. The case where the source electrode 18 and the drain electrode 19 are formed to make direct contact with the channel region has been exemplarily shown in this embodiment. However, as long as they can obtain ohmic contact with the channel region, they do not necessarily have to make direct contact to the channel region.

Second Embodiment

A second embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 5 shows a cross-sectional structure of a nitride semiconductor transistor according to the second embodiment. The description of the components shown in FIG. 5 that are the same as those shown in FIG. 1 will be omitted by retaining the same reference numerals.

Referring to FIG. 5, the transistor according to the second embodiment has a thin portion formed in a portion of the high resistive region 15 b located between the control region 15 a and each of the drain electrode 19 and the source electrode 18.

As the resistance of the high resistive region 15 b between the control region 15 a and the drain electrode 19 is higher, a greater amount of leakage current between the gate and the drain can be reduced. The resistance of the high resistive region 15 b increases with a decreasing thickness. Therefore, in the transistor of the second embodiment, the high resistive region 15 b located between the control region 15 a and the drain electrode 19 is etched to have a thickness of about 100 nm.

FIG. 6 shows the off-state breakdown voltage characteristics of the transistor according to the second embodiment. In FIG. 6, the gate voltage is set at 0 V. As shown in FIG. 6, as the high resistive region 15 b is thinner, the drain leakage current occurring in increasing the drain voltage is reduced. From this, it is found that the breakdown voltage property of the transistor is improved.

In the second embodiment, the high resistive region is thinned also between the control region and the source electrode. However, in order to decrease the leakage current between the gate and the drain to improve the breakdown voltage, it is sufficient that at least the high resistive region between the control region and the drain electrode is thinned.

Third Embodiment

A third embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 7 shows a cross-sectional structure of a nitride semiconductor transistor according to the third embodiment. The description of the components shown in FIG. 7 that are the same as those shown in FIG. 5 will be omitted by retaining the same reference numerals.

Referring to FIG. 7, in the transistor according to the third embodiment, the top of the third nitride semiconductor layer 15 is formed with not a hydrogen diffusion barrier film but a hydrogen diffusion film 17 for supplying hydrogen. It is sufficient that the hydrogen diffusion film 17 is deposited by, for example, a plasma enhanced CVD method and made of silicon nitride (SiN) containing equal to or more than 1×10²⁰/cm³ of hydrogen. The SiN film containing hydrogen releases hydrogen by a thermal treatment. Therefore, to a portion of the third nitride semiconductor layer 15 covered with the hydrogen diffusion film 17, hydrogen from the hydrogen diffusion film 17 is supplied during a thermal treatment for selectively inactivating impurities. Thus, in the portion of the third nitride semiconductor layer 15 covered with the hydrogen diffusion film 17, less or no hydrogen desorbs from the contained impurities, whereby the impurities are kept in inactivated state. On the other hand, in the portion thereof exposed from the opening of the third nitride semiconductor layer 15, the impurities contained are activated, whereby the control region 15 a having p-type conductivity can be selectively formed.

FIG. 8 shows the correlation between the temperature of the thermal treatment and the resistivity in the case where an Al_(0.15)Ga_(0.85)N layer doped with Mg to have a concentration of about 1×10¹⁹ cm³ is subjected to a thermal treatment. The thermal treatment was performed in a nitrogen atmosphere for 20 minutes. As shown in FIG. 8, in the case of forming the hydrogen diffusion film of SiN containing hydrogen, the resistivity of the Al_(0.15)Ga_(0.85)N layer slightly increases with an increasing temperature of the thermal treatment. This is probably because hydrogen is supplied from the hydrogen diffusion film and then the supplied hydrogen combined with Mg in the Al_(0.15)Ga_(0.85)N layer.

FIG. 9 shows the correlation between the hydrogen concentration in the hydrogen diffusion film and the resistivity of the Al_(0.15)Ga_(0.85)N layer when the Al_(0.15)Ga_(0.85)N layer doped with Mg is covered with the hydrogen diffusion film and then subjected to a thermal treatment. In FIG. 9, the temperature of the thermal treatment is set at 800° C. and the thermal treatment time is set at 20 minutes. As shown in FIG. 9, by setting the hydrogen concentration in the hydrogen diffusion film at 1×10²⁰ cm⁻³ or higher, Mg inactivation can be promoted to raise the resistivity. The SiN film containing an appropriate concentration of hydrogen can be formed easily by setting an appropriate film-formation condition for forming the SiN film by a plasma enhanced CVD method.

The case where part of the high resistive region 15 b is thinned like the second embodiment has been exemplarily shown in this embodiment, but the thickness of the high resistive region 15 b may be constant like the first embodiment. In the case where a thermal treatment step of exposing the transistor to a temperature of 400° C. or higher is not carried out after formation of the control region 15 a, the hydrogen diffusion film 17 may be removed after formation of the control region 15 a.

Fourth Embodiment

A fourth embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 10 shows a cross-sectional structure of a nitride semiconductor transistor according to the fourth embodiment. The description of the components shown in FIG. 10 that are the same as those shown in FIG. 7 will be omitted by retaining the same reference numerals.

Referring to FIG. 10, in the transistor according to the fourth embodiment, the first nitride semiconductor layer 13 of GaN is set to have a thickness of 10 nm, and a fourth nitride semiconductor layer 21 of undoped Al_(0.08)Ga_(0.92)N having a thickness of 1 μm is provided under the first nitride semiconductor layer.

FIG. 11 shows an energy band of the control region of the transistor according to the fourth embodiment. As shown in FIG. 11, the first nitride semiconductor layer 13 is interposed between the fourth nitride semiconductor layer 21 and the second nitride semiconductor layer 14 both of which are made of AlGaN and have wide band gaps. Therefore, the band gap has a concave occurring at the first nitride semiconductor layer 13, and a potential well is created in the conduction band and the valence band.

In the transistor of the fourth embodiment, at the interface between the first nitride semiconductor layer 13 and the fourth nitride semiconductor layer 21, the potential well is created in the valence band. Thus, holes injected from the control region 15 a into the channel region are confined by the potential well. This further efficiently increases a current flowing in the channel region by the holes injected into the channel region.

FIG. 12 shows the drain current and the gate voltage characteristics. As shown in FIG. 12, by providing the fourth nitride semiconductor layer 21, the threshold voltage becomes about 2 V. Therefore, even though a positive bias of 5 V or higher is applied to the gate electrode, a significant gate leakage current reduces and a maximum drain current of about 400 mA/mm is obtained.

In the fourth embodiment, the Al contents of the second nitride semiconductor layer and the fourth nitride semiconductor layer are not particularly limited. However, from the viewpoint of increasing the carrier concentration in the channel region, it is preferable that the Al content of the second nitride semiconductor layer be made higher than the Al content of the fourth nitride semiconductor layer. In addition, the electron concentration in the channel region becomes equal to the difference in the amount of spontaneous polarization between the fourth nitride semiconductor layer 21 and the second nitride semiconductor layer 14. Therefore, if the fourth nitride semiconductor layer 21 is set to have the same Al content as the second nitride semiconductor layer, the average electron concentration in the channel region can be almost zero when no bias is applied to the control region. This makes it possible to increase a current in the channel region according to the amount of holes injected from the control region into the channel region.

Although the case of forming the hydrogen diffusion film has been exemplarily shown in FIG. 10, the case of forming a hydrogen barrier film can also exert the same effects.

Fifth Embodiment

A fifth embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 13 shows a cross-sectional structure of a nitride semiconductor transistor according to the fifth embodiment. The description of the components shown in FIG. 13 that are the same as those shown in FIG. 7 will be omitted by retaining the same reference numerals.

Referring to FIG. 13, in the transistor of the fifth embodiment, the source electrode 18 is extended to the back surface of the substrate 11. A via hole is formed through the high resistive region 15 b, the second nitride semiconductor layer 14, the first nitride semiconductor layer 13, and the buffer layer 12 and to etch part of the substrate 11 of silicon, and the via hole is filled with a conductive material to form a via plug 23. A via-hole metal film 24 of Al or the like is formed on a portion of the substrate 11 located at the bottom surface of the via hole, and the substrate 11 and the via plug 23 are in ohmic contact with each other.

The via plug 23 and the source electrode 18 are electrically connected by a metal interconnect 26 formed through a passivation film 25 of SiN covering the top surface of the transistor. The back surface of the substrate 11 is formed with a backside electrode 27 made of, for example, a stacked film of AuGeSb, Cr, NiCr, and Ni, and the source electrode 18 can be grounded from the back surface of the substrate 11.

Although the case where the source electrode of the transistor of the third embodiment is extended to the back surface of the substrate has been exemplarily shown in FIG. 13, the source electrode of the transistor of another embodiment can also be extended to the back surface of the substrate in the same manner.

In each of the embodiments, description has been made of the normally-off type transistor. Alternatively, the p-type impurity concentration in the control region, the thickness and the Al composition of the AlGaN layer serving as the second nitride semiconductor layer, and the like can be adjusted to form a normally-on type transistor. Also in the normally-on type transistor, holes are injected from the control region into the channel region, whereby not only an operating current can be increased but also a positive bias as high as 5 V or more can be applied to the gate electrode. This offers expectations for a significant effect such as facilitation of design of a gate drive circuit of the nitride semiconductor transistor.

As described above, with the nitride semiconductor transistor and its fabrication method according to the present invention, a nitride semiconductor transistor can be provided which has a large operating current and an excellent switching characteristic and which is easily designed to operate as a normally-off transistor. Accordingly, for example, the present invention is useful for a nitride semiconductor transistor such as a power transistor with a high breakdown voltage, and for its fabrication method. 

1. A nitride semiconductor device comprising: a first nitride semiconductor layer formed over a substrate; a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a wider band gap than the first nitride semiconductor layer; a third nitride semiconductor layer which is formed on the second nitride semiconductor layer and which includes a control region with a p-type conductivity and a high resistive region formed around the control region and having a higher resistance than the that of the control region; and a gate electrode formed on the control region.
 2. The device of claim 1, further comprising a source electrode and a drain electrode formed on both sides of the gate electrode, respectively, and electrically connected to a channel region, wherein the electric conductivity between the source electrode and the drain electrode is controlled by injecting holes into the channel region in the manner in which a forward bias voltage with respect to the channel region is applied to the control region.
 3. The device of claim 2, wherein groves are formed at source and drain regions through the third and second nitride semiconductor layers, and the source electrode and the drain electrode cover the grooves.
 4. The device of claim 1, wherein the third nitride semiconductor layer contains a p-type impurity, and the p-type impurity contained in the control region is activated, and the p-type impurity contained in the high resistive region is inactivated by combining with hydrogen.
 5. The device of claim 1, wherein the control region contains holes more than electrons contained in two-dimensional electron gas induced at the interface between the first nitride semiconductor layer and the second nitride semiconductor layer in which the third nitride semiconductor layer is not covered.
 6. The device of claim 4, further comprising a hydrogen diffusion barrier film for reducing hydrogen desorption from the p-type impurity in the high resistive region of the third nitride semiconductor layer during annealing the third nitride semiconductor layer in order to activate the p-type impurity contained in the control region, the hydrogen diffusion barrier film being formed on the third nitride semiconductor layer, wherein the hydrogen diffusion barrier film has an opening for exposing at least part of the control region.
 7. The device of claim 6, wherein the hydrogen diffusion barrier film is made of aluminum nitride, aluminum oxide, or silicon nitride.
 8. The device of claim 4, further comprising a hydrogen diffusion film for providing hydrogen to inactivate the p-type impurity contained in the high resistive region during annealing the third nitride semiconductor layer, the hydrogen diffusion film being formed on the third nitride semiconductor layer wherein the hydrogen diffusion film has an opening for exposing at least part of the control region.
 9. The device of claim 8, wherein the hydrogen diffusion film is a silicon nitride film containing equal to or more than 1×10²⁰/cm³ of hydrogen.
 10. The device of claim 1, wherein the first nitride semiconductor layer is made of GaN, the second nitride semiconductor layer is made of Al_(x)Ga_(1-x)N (0<x≦1), and the third nitride semiconductor layer is made of Al_(y)Ga_(1-y)N (0≦y≦1).
 11. The device of claim 1, further comprising a fourth nitride semiconductor layer for forming a potential barrier in a valence band to the first nitride semiconductor layer, the fourth nitride semiconductor layer being formed between the substrate and the first nitride semiconductor layer.
 12. The device of claim 11, wherein the first nitride semiconductor layer is made of GaN, the second nitride semiconductor layer is made of Al_(x)Ga_(1-x)N (0<x≦1), the third nitride semiconductor layer is made of Al_(y)Ga_(1-y)N (0≦y≦1), and the fourth nitride semiconductor layer is made of Al_(z)Ga_(1-z)N (0<z≦1).
 13. The device of claim, 1, wherein the high resistive region located between the gate electrode and the drain electrode has a thinner portion than the control region.
 14. A method for fabricating a nitride semiconductor device, comprising: the step (a) of forming, over a substrate, a first nitride semiconductor layer, a second nitride semiconductor layer, and a third nitride semiconductor layer in this order from bottom to top, the second nitride semiconductor layer having a wider band gap than the first nitride semiconductor layer; the third nitride semiconductor layer containing a p-type impurity combined with hydrogen; the step (b) of forming a control region having p-type conductivity by hydrogen desorption from p-type impurities in a portion of the third nitride semiconductor layer; the step (c) of forming a gate electrode on the control region; and the step (d) of forming a source electrode and a drain electrode on both sides of the gate electrode, respectively, the source electrode and the drain electrode being electrically connected to a channel region.
 15. The method of claim 14, wherein the step (b) includes: the substep (b1) of forming, on the third nitride semiconductor layer, a hydrogen diffusion barrier film for reducing hydrogen penetration, and then forming an opening at a location to be formed with the control region; and the substep (b2) of annealing, after the step (b1), the third nitride semiconductor layer.
 16. The method of claim 15, wherein the hydrogen diffusion barrier film is made of aluminum nitride, aluminum oxide, or silicon nitride.
 17. The method of claim 14, wherein the step (b) includes: the substep (b1) of forming, on the third nitride semiconductor layer, a hydrogen diffusion film for providing hydrogen for the third nitride semiconductor by annealing, and then forming an opening at a location to be formed with the control region; and the substep (b2) of annealing, after the step (b1), the third nitride semiconductor layer.
 18. The method of claim 17, wherein the hydrogen diffusion film is a silicon nitride film containing equal to or more than 1×10²⁰/cm³ of hydrogen. 